1. Field of the Invention
This invention relates to a vehicle navigation system for providing the vehicle operator or his or her assistant with information regarding the route from a given initial location to a given destination.
2. Description of the Prior Art
There have been proposed various so-called route guidance type vehicle navigation systems equipped with an appropriate display device for providing the vehicle operator etc. with information regarding the route to be followed from the current vehicle location to a given destination.
Typically, the vehicle navigation systems of this type are arranged to store road map information as digital data, calculate the route between the current location and the destination upon receiving the two locations as input, and providing the operator with route guidance information at appropriate times as the vehicle navigates the route. The guidance information is output shortly before the vehicle reaches each intersection, for example, and instructs the operator as to whether he or she should continue straight ahead, turn right or turn left.
If a vehicle being guided by such a prior art route guidance navigation system should stray from the route along which it is being guided in the course of the series of route guidance steps executed in accordance with the calculated route, a fresh calculation is made for determining the route between the off-route location of the vehicle at that time and the destination and a different series of route guidance steps are executed for guiding the vehicle from the location to which it has strayed to the destination.
The deviation of the vehicle from the calculated route may occur accidentally owing to an operator error or may be deliberate, as when the operator pulls into a service station or a restaurant parking lot. In the latter case, the operator almost always eventually returns to the original route.
As the prior art systems invariably have to recalculate the route to the destination even when the operator only temporarily deviates from the calculated route for some reason or other, they frequently end up conducting unnecessary calculations that involve the operator and thus increase the number of tasks the operator has to perform.